Method of determining the position of a target using transmitters of opportunity

ABSTRACT

A method of determining the position of a target comprising the steps of: providing a transmitter or a plurality of transmitters to transmit a signal to the target, providing a receiver or a plurality of receivers to receive signals reflected from said target. Determining the time of arrival information of said reflected signal at the or each receiver; using information pertaining to the position of the or each receiver and/or the or each transmitter and with the information obtained determining the target position. Preferably the signal includes a modulated or coded portion unique to the transmitter; and said signal portion is associated with the or each particular transmitter by virtue of said signal portion. The signal maybe a mobile phone transmission, DAB, digital TV, digital Radio or digital satellite transmissions.

The use of radars to detect the position of moving objects is wellknown. It has also been previously proposed to detect the position of atarget using so called “transmission of opportunity” wherein receiversdetect transmissions, reflected from a target, originating from atransmitter. However in such systems, a direct line of sight of thereceiver to the transmitter must also be provided (or a cable connectionis needed) in order to create a reference in the receivers which canthen be used for correlation with the delayed signal arriving viareflection from a target.

In certain telecommunication systems, transmitters use pre-stored codesas part of their modulation protocol. The inventors have determined thatthese codes can be advantageously used as templates for a correlatorwhen correlating signals from a receiver reflected from a target inorder to provide timing information to assist target location. Thisenables a method of detecting target positional information whichobviates the need to have a direct line of sight from the receiver

The invention comprises in a communication system wherein pre-storedcodes are used in transmissions as part of the communication protocol, amethod of determining the position of a target comprising the steps of:providing a transmitter to transmit a signal to the target, providing aplurality of receivers, in communication with each other, to receivesignals reflected from said target, the receivers being time or phasesynchronised; determining the time of arrival information of said codeof reflected signal at each receiver by continuously correlating saidreceived signal with the pre-stored codes in the receiver; usinginformation pertaining to the position of each receiver and with theinformation obtained, determining the target position.

The invention also includes in a communication system wherein pre-storedcodes are used in transmissions as part of the communication protocol, amethod of determining the position of a target comprising the steps of:providing a plurality of transmitters to transmit each a signal to thetarget, said transmitters being time or phase synchronised, providing areceiver to receive signals reflected from said target and wherein saidsignals are sent out pre-set times known to the receiver; correlatingsaid received signal with the pre-stored codes in each receiver toidentify the transmitter of a particular signal; the receiverdetermining the time delay in receiving said signal by subtracting thetime of arrival of a pre-stored code of each reflected signal from eachtransmitter from the known time the pre-stored code was sent, usinginformation known to the receiver pertaining to the position of eachtransmitter and the with the timing information obtained, determiningthe target position

Transmitters in many current communications systems are using pre-storedcodes as part of their modulation protocol (e.g. for the purposes ofchannel equalisation or signal identification). The invention makes useof these pre-stored codes in a number of ways to detect the position ofa target, where signals from transmitter(s) are reflected from thetarget to receivers. At the receivers detection of the codes bycorrelation allows the receiver to detect when the embedded code isreceived. This along with other information, such as transmitter orreceiver location, enables target location to be determined. One mainembodiment uses known information of when the codes are sent to provideuseful timing information; this is because some codes are sent out atpre-determined time from a transmitter, these times also being known bya receiver.

In some systems, the codes will also include have a uniqueidentification of a particular transmitter and a receiver may look upthe location; i.e. the base station identity, contained within thesignal can also be used in order to help sort out where eachtransmission came from. This eliminates any identification problems withrespect to different signals from different base stations containing thesame information producing correlations. In known telecommunicationsystems it is known to includes a modulated or coded portion unique tothe transmitter.

EXAMPLE 1

In this example there is a single transmitter and a number of receivers.An example may be a transmitter used in mobile communications. Thetransmitted signal includes one or more codes which are used in theprotocol(s). A signal is transmitted and will be reflected from a targetsuch as an aircraft to a plurality of receivers. Each receiver has thecode pre-stored and continuously correlates this code with the receivedsignal. When there is a peak in the correlation, the time T(i) is notedby each receiver i. These times are then sent to a processor, which maybe located in one of the receivers. The processor used the times thatthe code was received and known information as to the positions of thereceivers to calculate the position of the target. It is to be notedthat such a system relies upon the code signal portion being definedwithin the protocol standard for the transmission type and as such iswell defined and does not change. Also it is worth mentioning that theabsolute time taken between transmission and reception of signal doesnot have to be known, just the (relative) times at each receiver whenthe reflected signal is received is important.

In other words the position where the correlation peak occurs willindicate the time at which that signal was received with respect to thereference template. This is especially useful when time difference ofarrival techniques are used because in this case the absolute time thatthe transmission was sent is irrelevant, it is the relative time betweensignals (sent from the same location) arriving at different receiverlocations that reveals the target position. It would be clear to aperson skilled in the art how the resultant data can be used todetermine target position e.g. triangulation.

Because the codes are known to be within the waveform by design, it isnot now necessary to record those codes via a direct link to thetransmitter. The codes are pre-ordained and as such may bepre-programmed into every receiver, thereby allowing the crosscorrelation process to occur irrespective of the data content of thespecific transmitted signal. The receivers need to be synchronised.

EXAMPLE 2

In this example there are a plurality of transmitters and a singlereceiver. The transmitters are synchronised. Each transmitter will senda signal which is received by the receiver. The receiver will againcorrelate incoming signals with a pre-stored code, and will make a noteof the time that the prestored code is received for each transmitter.However in this case the absolute time between transmission and thereflected signal from the target being received is determined. Themethod makes use of the fact that embedded codes used in a protocol isin a fixed (known position) within the transmitted signal and thetransmitter transmits at pre-set times (known to the receiver also).Thus if clocks of the transmitters are synchronised with that of thereceiver the absolute time difference between signal sent and received(reflected via the target) can readily be determined by the receiver foreach transmitter signal. With known data on the location of thetransmitters the receiver can determine the target location.

In the example of a modern 3G cell phone systems use fully timesynchronised networks so that every base station has a pre determinedseries of time and frequency slots when it transmits. If the receiversare then synchronised to the cellular network they know which basestation has transmitted to within the accuracy of the synchronisationscheme 1 or 2 seconds. Now when the correlation process determines thetime of arrival of the transmitted signal reflected from the target (inthe same way as before) it can subtract the known transmission time toreveal the time of flight of the reflected signal. This time of flightinformation places the target on a locus that links all the positionswhere the range of the target with respect to the transmitter andreceiver positions is a constant. To resolve the position on this locusof the target a second locus, with a different transmitter position isneeded. Now the intersection of these loci defines the target position.Clearly, where signal reflections from multiple transmitter are receivedthe estimate of target position becomes more accurate.

In a preferred embodiment of both examples, the signal from eachtransmitter preferably has unique identifying data so the receiver candiscriminate the reflected signals in respect of which transmitter theycame from. This reduces the number of parameters required such as thenumber of receivers/and or transmitter required for input to calculateposition. Furthermore, if the receiver has pre-stored the geometriclocation of the identified transmitters i.e. the relative positions ofthe transmitters are known, the position of the target can be determinedmore readily.

Of course there are a number of different permutations and combinationswhich fall within the scope of the invention which would be clear to theskilled person.

In some instances it may be necessary where the transmitter iscontinuously sending out signals, for the receivers to discriminatebetween successive signals so that they are assessing the same signal,i.e. sent out at the same time.

Naturally the system may comprise of a plurality of receivers and aplurality of transmitters. Although the calculation of such a hybridsystem would be more involved it would be clear to the skilledmathematician how the fdata can be used.

Furthermore the inventor has determined that current mobiletelecommunications networks lend themselves very well to implement theabove methods. The system is well adapted to use standard mobiletelephone transmissions because of the aforementioned pre-stored codesused in mobile phone transmissions. For example in mobile phonemodulation protocol there are included unique base stationidentification data and so called “training sequences” which areadvantageously used in determination of target position. A GSM networkoperates typically with 200 KHz channels. Training codes (also calledmid-amble) which are 26 bits long occur in every package i.e. every 660microseconds. It is these codes that are used as the correlatorreference in each receiver.

In one example of the invention standard telephone receivers are usedand the received signal to an A/D converter occupying 200 KHz bandwidthis demodulated and the data package segmented into its training sequenceand base station ID sequence. The training sequence is fed forward forcorrelation with the pre-stored training sequence in order to determinethe time of arrival of the transmission. The base station identificationis decoded and the bases station position can be determined (viapre-surveyed information for example), if this is needed for aparticular implementation.

1. In a communication system wherein pre-stored codes are used intransmissions as part of the communication protocol, a method ofdetermining the position of a target comprising the steps of: a)providing a transmitter or to transmit a signal to the target, b)providing a plurality of receivers, in communication with each other, toreceive signals reflected from said target, the receivers being time orphase synchronised; c) determining the time of arrival information ofsaid code of reflected signal at each receiver by continuouslycorrelating said received signal with the prestored codes in thereceiver; d) using information pertaining to the position of eachreceiver and with the information obtained from step c), determining thetarget position.
 2. In a communication system wherein pre-stored codesare used in transmissions as part of the communication protocol, amethod of determining the position of a target comprising the steps of:a) providing a plurality of transmitters to transmit each a signal tothe target, said transmitters being time or phase synchronized; b)providing a receiver to receive signals reflected from said target andwherein said signals are set out pre-set times known to the receiver; c)correlating said received signal with the pre-stored codes in eachreceiver to identify the transmitter of a particular signal; d) thereceiver determining the time delay in receiving said signal bysubtracting the time of arrival of a pre-stored code of each reflectedsignal from each transmitter from the known time the pre-stored code wassent, e) using information known to the receiver pertaining to theposition of each transmitter and with the timing information obtainedfrom step c), determining the target position.
 3. A method as claimed inclaim 1 wherein transmitted signals includes a modulated or codedportion unique to the transmitter which is used to identify thetransmitter and hence its location for step (e).
 4. A method as claimedin claim 1, wherein all the receivers communicate said time of arrivalinformation and receiver identification to central unit.
 5. A method asclaimed in claim 4 wherein said central unit is one of the receivers. 6.A communication system having a plurality of transmitters and aplurality of receivers, using combined information obtained via themethods of claim 1 to determine the location of a target.
 7. A method asclaimed in claim 1, wherein said signal is a mobile phone transmission,DAB, digital TV, digital Radio or digital satellite transmission.
 8. Amethod as claimed in claim 1, wherein said signal includes the mid-ambleor a portion thereof or any unique sequence of data that is known to thereceiver before transmission.